Aluminum-boron solar cell contacts

ABSTRACT

Formulations and methods of making solar cells are disclosed. In general, the invention provides a solar cell comprising a contact made from a mixture wherein, prior to firing, the mixture comprises at least one aluminum source, at least one boron source, and about 0.1 to about 10 wt % of a glass component. Within the mixture, the overall content of aluminum is about 50 wt % to about 85 wt % of the mixture, and the overall content of boron is about 0.05 to about 20 wt % of the mixture.

RELATED APPLICATIONS

This application is a continuation of application Ser. No. 11/384,838filed Mar. 20, 2006 which is incorporated herein by reference.

FIELD OF THE INVENTION

This invention relates to an aluminum-boron conductor formulation madefrom conductive aluminum particles, boron containing sources, inorganicadditives, and glass frit dispersed in an organic system. Theformulations are mainly screen-printable and suitable for use in thefabrication of photovoltaic devices. These formulations could also beapplied by other methods such as spraying, hot melt printing, ink jetprinting, pad printing and tape lamination techniques with suitablemodifications of organics.

BACKGROUND

Solar cells are generally made of semiconductor materials, such assilicon (Si), which convert sunlight into useful electrical energy. Asolar cell contact is in generally made of thin wafers of Si in whichthe required PN junction is formed by diffusing phosphorus (P) from asuitable phosphorus source into a P-type Si wafer. The side of thesilicon wafer on which sunlight is incident is generally coated with ananti-reflective coating (ARC) to prevent reflective loss of sunlight.This ARC increases the solar cell efficiency. A two dimensionalelectrode grid pattern known as a front contact makes a connection tothe N-side of silicon, and a coating of predominantly aluminum (Al)makes connection to the P-side of the silicon (back contact). Further,contacts known as silver rear contacts, made out of silver orsilver-aluminum paste are printed and fired on the N-side of silicon toenable soldering of tabs that electrically connect one cell to the nextin a solar cell module. These contacts are the electrical outlets fromthe PN junction to the outside load.

Conventional pastes for solar cell contacts contain lead frits.Inclusion of PbO in a glass component of a solar cell paste has thedesirable effects of (a) lowering the firing temperature of pastecompositions, (b) facilitating interaction with the silicon substrateand, upon firing, helping to form low resistance contacts with silicon.For these and other reasons PbO is a significant component in manyconventional solar cell paste compositions. However, in light ofenvironmental concerns, the use of PbO (as well as CdO), in pastecompositions is now largely avoided whenever possible. Hence a needexists in the photovoltaic industry for lead-free and cadmium-free pastecompositions, which afford desirable properties using lead-free andcadmium-free glasses in solar cell contact pastes.

Presently, a typical solar cell silicon wafer is about 200-300 micronsthick, and the trend is toward thinner wafers. Because the wafer cost isabout 60% of the cell fabrication cost, the industry is seekingever-thinner wafers, approaching 150 microns. As the wafer thicknessdecreases, the tendency toward bowing (bending) of the cell due to thesintering stress increases, which is generated by the great differencein the thermal coefficients of expansion (TCE) between aluminum(232×10⁻⁷/° C. @ 20-300° C.) and silicon, (26×10⁻⁷/° C. @ 20-300° C.).

Known methods of mitigating silicon wafer bowing include reduction ofaluminum content during screen-printing that causes incomplete formationof Back Surface Field (BSF) layers and requires a higher firingtemperature to achieve the same results. Chemical (acid) etching hasbeen used to remove the Al—Si alloy that forms after firing the Aluminumpaste. This is just another step in the manufacturing process that leadsto additional cost.

Another approach is to use additives to reduce the thermal expansionmismatch between the Al layer and the silicon wafer. However, a drawbackis a reduction in rear surface passivation quality and a concomitantreduction in solar cell performance. Partial covers, where only aportion of the back side of the wafer is coated with aluminum, have beenused on the back surface field to counteract bowing, which causes areduction in cell performance.

Finally, another conventional way to reduce or eliminate bowing iscooling a finished solar cell from room temperature to ca. −50° C. forseveral seconds after firing. With such plastic deformation of the Al—Sipaste matrix, bowing is largely eliminated, but this represents anadditional process step, and there is a high danger of breakage fromthermal stress.

Hence a need exists in the photovoltaic industry for a low-bow,high-performance aluminum back surface field in a solar cell contact, amethod of making such a contact, and the Al paste from which such a BSFis formed.

SUMMARY OF THE INVENTION

The present invention provides an aluminum-boron paste for applicationto a silicon solar cell having a p+ and n+ layer for the formation of aback-surface-field (BSF) and an emitter. The boron-doped aluminumcontact formed by firing the paste eliminates or minimizes bowing ofultra thin silicon wafers, thereby improving reliability and electricalperformance of solar cells made therewith, as measured by low seriesresistance (R_(S)) and high shunt resistance (R_(sh)) high efficiency(η) and high fill factor (FF), as well as reducing breakage.

Generally, the present invention includes a solar cell comprising acontact. The contact is made from a mixture wherein prior to firing, themixture comprises at least one aluminum source, at least one boronsource, and about 0.1 to about 10 wt % of a glass component. The contentof aluminum is about 50 wt % to about 85 wt % of the mixture, and thecontent of boron is about 0.05 to about 20 wt % of the mixture.

Another embodiment of the invention is a solar cell comprising a siliconwafer, aluminum, and boron, wherein the combined concentration ofaluminum and boron (Al+B) at a depth of about 0 to about 5 microns inthe silicon wafer is about 10¹⁸ to about 10²⁰ atoms per cubic centimeter(cm³).

The compositions and methods of the present invention overcome thedrawbacks of the prior art by optimizing interaction, bonding, andcontact formation between back contact (BSF) components, typicallysilicon with Al through a properly formulated aluminum-boron paste. Aconductive paste containing aluminum, boron, and a glass component, isprinted on a silicon substrate, and fired to fuse the glass, sinter themetal, and provide aluminum doping into the silicon wafer to a depth ofseveral microns. Upon firing, for a back contact, a p+ layer is formed,which is overlaid by an Al—Si eutectic layer, and which in turn isoverlaid by aluminum layer which could contain boron.

The foregoing and other features of the invention are hereinafter morefully described and particularly pointed out in the claims, thefollowing description setting forth in detail certain illustrativeembodiments of the invention, these being indicative, however, of but afew of the various ways in which the principles of the present inventionmay be employed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing the combined concentration of aluminum andboron as a function of depth into a silicon wafer in a solar cellcontact in accordance with the invention.

DETAILED DESCRIPTION OF THE INVENTION

Broadly, the invention provides a solar cell comprising a contact. Thecontact is made from a mixture wherein prior to firing, the mixturecomprises at least one aluminum source, at least one boron source, andabout 0.1 to about 10 wt % of a glass component. The content of aluminumis about 50 wt % to about 85 wt % of the mixture, and the content ofboron is about 0.05 to about 20 wt % of the mixture.

Another embodiment of the invention is a solar cell comprising a siliconwafer, aluminum, and boron, wherein the combined concentration ofaluminum and boron (Al+B) at a depth of about 0 to about 5 microns inthe silicon wafer is about 10¹⁸ to about 10²⁰ atoms per cubic centimeter(cm³).

Another embodiment of the invention is a solar cell comprising a siliconwafer, aluminum, and boron, wherein the combined concentration ofaluminum and boron (Al+B) at a depth of about 0 to about 5 microns inthe silicon wafer is about 10¹⁸ to about 10²⁰ atoms per cubic centimeter(cm³).

An aluminum back contact makes contact with both Si and the Ag/Al rearcontact layer. In a back contact, the metal component preferablycomprises aluminum and boron, and the glass component may be one ofseveral types. Bismuth based glasses and alkali titanium silicateglasses each have certain advantages over the prior art when used in asolar cell back contact.

Broadly, thick film pastes containing aluminum and glass frit are usedto make back contacts for silicon-based solar cells to conduct to anexternal load the current generated by exposure to light. While thepaste is generally applied by screen-printing, methods such asextrusion, pad printing, ink jet printing, and hot melt printing mayalso be used. Further with suitable organics modifications the mixtureof the present invention could be applied by tape lamination techniques.Solar cells with screen-printed front contacts are fired to relativelylow temperatures (550° C. to 850° C. wafer temperature; furnace settemperatures of 650° C. to 1000° C.) to form a low resistance contactbetween the P-side of an aluminum doped silicon wafer and an aluminumbased paste. Methods for making solar cells are also envisioned herein.

Aluminum- and glass-containing back contacts are used to form lowresistance ohmic contacts on the back side of the solar cell due tolarge area melting and re solidification of Al doped (p⁺) epitaxiallygrown Si layer which increases the solar cell performance due toimproved back surface field. For optimum performance a thick p⁺ re-grownregion is believed to be ideal. It is also believed that the rejectionof metallic impurities from the epitaxially growing p⁺ layer leads tohigh carrier lifetimes. These two factors are believed to increase theopen circuit voltage, and more importantly, the open circuit voltagefalls only slightly as the bulk resistivity increases. Therefore solarcell performance improves due to the formation of a substantialepitaxially re-grown p⁺ layer in the Al back contact.

For a back contact, upon firing, a p⁺ layer forms on the underlyingsilicon by liquid-phase epitaxy. This occurs during the resolidificationof the aluminum-silicon (Al—Si) melt. High-bismuth lead-free andcadmium-free glasses allow low firing temperatures in making frontcontacts owing to their excellent flow characteristics relatively at lowtemperatures. Alkali-titanium-silicate glasses are another route toattain lower firing temperatures. While lead-glasses are often avoidedfor environmental reasons, they are sometimes used because they are theonly route at present to certain properties, such as extremely lowmelting and wetting glasses. Relatively high-silicon, low bismuthlead-free and cadmium-free glasses provide suitable properties for backcontacts, without excessive interaction with backside Si. Similarly,high-bismuth lead-free and cadmium-free glasses allow the formation ofsuitable lead-free silver rear contacts on backside Si with optimalinteraction with both Si and back contact Al layer.

Broadly construed, the inventive pastes comprise aluminum, boron, andglass. Each ingredient is detailed hereinbelow.

Paste Glasses.

The pastes comprise about 0.1 to about 10 wt %, preferably 0.2 to about5 wt % of a glass component. The glass component comprises, prior tofiring, one or more glass compositions. Each glass composition comprisesoxide frits including, in one embodiment, Bi₂O₃, B₂O₃ and SiO₂. Inanother embodiment, the glass composition comprises an alkali oxide,TiO₂, and SiO₂. In a third embodiment, the glass composition comprisesPbO. In particular, in various embodiments of the present invention,glass compositions for back contacts may be found in Tables 1-3. Theentry “20 trivalent oxides” means one or more trivalent oxides of anelement selected from the group consisting of Al, Ga, In, Sc, Y, and anelement having an atomic number of from 57 to 71. In formulating thepastes, the glass frits typically have particle sizes of about 0.5 toabout 10 microns, although other particle sizes may be used as known inthe art.

Looking to Tables 1-3, more than one glass composition can be used, andcompositions comprising amounts from different columns in the same tableare also envisioned. If a second glass composition is used, theproportions of the glass compositions can be varied to control theextent of paste interaction with silicon and hence the resultant solarcell properties, and to control the bowing of the silicon wafer. Forexample, within the glass component, the first and second glasscompositions may be present in a weight ratio of about 1:20 to about20:1, and preferably about 1:5 to about 5:1. The glass componentpreferably contains no lead or oxides of lead, and no cadmium or oxidesof cadmium. However, in certain embodiments where the properties of PbOcannot be duplicated, such embodiments advantageously comprise PbO. Anentry such as “Li₂O+Na₂O+K₂O” means that the total content of Li₂O andNa₂O and K₂O falls within the specified ranges.

TABLE 1 Oxide frit ingredients for bismuth-based back contact glasses inmole percent. Glass Composition Ingredient I II III Bi₂O₃ 5-85 10-75 12-50 B₂O₃ + SiO₂ 5-75 15-75  34-71 Li₂O + Na₂O + K₂O 0-40 5-30 10-30 20trivalent oxides 0-25 0-20  3-10 ZnO 0-55 0-20  0-12 Sb₂O₅ + Nb₂O₅ 0-400-30  0-20 TiO₂ + ZrO₂ 0-20 0-10 1-6

TABLE 2 Oxide frit ingredients for alkali-titanium-silicate back contactglasses in mole percent. Glass Composition Ingredient IV V VI Li₂O +Na₂O + K₂O 5-55 15-50 30-40 TiO₂ 2-26 10-26 15-22 B₂O₃ + SiO₂ 5-75 25-7030-52 V₂O₅ + Sb₂O₅ + P₂O₅ 0-30 0.25-25    5-25 MgO + CaO + BaO + SrO0-20  0-15  0-10 F 0-20  0-15  5-13

TABLE 3 Oxide frit ingredients for lead based back contact glasses inmole percent. Glass Composition Ingredient VII VIII IX PbO 15-75  25-6650-65 B₂O₃ + SiO₂ 5-75 20-55 24-45 ZnO 0-55 0.1-35  0.1-25  Li₂O +Na₂O + K₂O 0-40  0-30  0-10 TiO₂ + ZrO₂ 0-20  0-10 0.1-5   20 trivalentoxides 0-25 0.1-20   1-10

In a preferred embodiment the glass component comprises: about 12 toabout 50 mole % Bi₂O₃; about 25 to about 65 mole % SiO₂; about 5 toabout 15 mole % B₂O₃; about 4 to about 26 mole % K₂O; TiO₂, wherein thecontent of TiO₂ does not exceed about 10 mole %; and an oxide of anelement selected from the group Li, Na, K, Sb and combinations thereof,provided the combined total of such oxides does not exceed about 40 mol%, preferably at least about 1 mol % of the combination. In a preferredembodiment containing alkali oxides the glass component comprises about1 to about 15 mole % Li₂O, about 8 to about 25 mole % Na₂O, about 3 toabout 25 mole % K₂O, about 8 to about 22 mole % TiO₂, about 25 to about50 mole % SiO₂, about 2 to about 18 mole % V₂O₅, and about 0.25 to about5 mole % P₂O₅, and may further comprise fluoride, not to exceed about 20mol %.

In another preferred embodiment, the composition may comprise one ormore of the following, so long as the content of the following oxidesdoes not exceed the indicated amount in mol % Li₂O (15%), Na₂O (25%),K₂O (25%), TiO₂ (22%), SiO₂ (60%), V₂O₅ (18%), the sum of(Sb₂O₅+V₂O₅+O₅) (25%), and F (15%)

The most preferred embodiments are those using lead free and cadmiumfree glasses discussed above. However, when properties unattainable byother than leaded glasses are required, then the glass component maycomprise one or more of the following, so long as the content of thefollowing oxides does not exceed the indicated amount in mol % PbO(65%), SiO₂ (30%), B₂O₃ (30%), ZnO (25%), and trivalent oxides ofelements selected from the group consisting of Al, Ga, In, Sc, Y, La(25%), and (TiO₂+ZrO₂) (5%), provided that the total of (B₂O₃+SiO₂) doesnot exceed 45%. The lead-containing glass components may furthercomprise about 0.1 to about 8 mol % Al₂O₃.

Metal Component.

In a solar cell contact, the metal must be conductive. In a backcontact, the metal component comprises aluminum. The aluminum metalcomponent may come in any suitable form, including aluminum metalpowder, an alloy of aluminum, an aluminum salt, and organometallicaluminum, an oxide of aluminum, and an aluminum-containing glass. Thealuminum particles used in the paste may be spherical, flaked, orprovided in a colloidal suspension, and combinations of the foregoingmay be used. In formulating the pastes, the metal powders typically haveparticle sizes of about 0.1 to about 40 microns, preferably less than 10microns. For example the paste may comprise about 80 to about 99 wt %spherical aluminum particles or alternatively about 75 to about 90 wt %aluminum particles and about 1 to about 10 wt % aluminum flakes.Alternatively the paste may comprise about 75 to about 90 wt % aluminumflakes and about 1 to about 10 wt % of colloidal aluminum, or about 60to about 95 wt % of aluminum powder or aluminum flakes and about 0.1 toabout 20 wt % of colloidal aluminum. Suitable commercial examples ofaluminum particles are available from Alcoa, Inc., Pittsburgh, Pa.;Ampal Inc., Flemington, N.J.; and ECKA Granulate GmbH & Co. KG, ofFürth, Germany.

An alloy of aluminum may be used, including those comprising aluminumand a metal selected from the group consisting of boron, silicon,gallium, indium, antimony, and magnesium. In one embodiment, the alloyprovides both aluminum and boron, namely, as an aluminum-boron alloy,which comprises about 60 to about 99.9 wt % aluminum and about 0.1 toabout 40 wt % boron. In a preferred embodiment an aluminum-boron alloycontaining 0.2 weight % B could be used for up to 98 wt % of the pastemixture to provide both aluminum and boron to the mixture. In yetanother preferred embodiment, one or more of the alloys Al—Si, Al—Mg,Al—Ga, Al—In, Al—Sb, Al—Sn, and Al—Zn may constitute up to about 50 wt %of the mixture.

The boron source may boron metal powder, an alloy of boron, a salt ofboron, boric acid, organometallic boron, an oxide of boron, andboron-containing glass. Combinations of the foregoing may be used. Inparticular, the boron source may be sodium borate, calcium borate,potassium borate, magnesium borate, B₂O₃ containing glass andcombinations thereof. In a preferred embodiment, an organometallic boronsolution could be used to provide boron, wherein such boron does notexceed about 5 wt % of the paste mixture.

The Al—B pastes herein can be used to form a p⁺ BSF several micronsthick with an active peak doping concentration of about 10¹⁹ to about10²⁰ atoms per cm³; that is, one or two orders of magnitude higher thanis achievable with conventional Al paste (e.g., on the order of 10¹⁸atoms per cm³). This new Al—B paste uses the higher solid solubility ofboron in Si and to increase the dissolution of Al into Si. ThereforeAl—B paste makes it possible to deposit thinner paste layers for BSFformation, reduce bowing, and exploit the gettering properties ofaluminum to improve minority carrier lifetime in solar cells withoutlosing good ohmic contact properties of the Al—Si layer. FIG. 1 showsmeasured p⁺ carrier concentration in silicon after Al—B alloying processusing the spreading resistance measurement. The spreading resistancedata shows variation on p⁺ carrier concentration due to roughness causedby the Al—B alloying process.

Organic Vehicle.

The pastes herein include a vehicle or carrier which is typically asolution of a resin dissolved in a solvent and, frequently, a solventsolution containing both resin and a thixotropic agent. The organicsportion of the pastes comprises (a) at least about 80 wt % organicsolvent; (b) up to about 15 wt % of a thermoplastic resin; (c) up toabout 4 wt % of a thixotropic agent; and (d) up to about 2 wt % of awetting agent. The use of more than one solvent, resin, thixotrope,and/or wetting agent is also envisioned. Although a variety of weightratios of the solids portion to the organics portion are possible, oneembodiment includes a weight ratio of the solids portion to the organicsportion from about 20:1 to about 1:20, preferably about 15:1 to about1:15, and more preferably about 10:1 to about 1:10.

Ethyl cellulose is a commonly used resin. However, resins such as ethylhydroxyethyl cellulose, wood rosin, mixtures of ethyl cellulose andphenolic resins, polymethacrylates of lower alcohols and the monobutylether of ethylene glycol monoacetate can also be used. Solvents havingboiling points (1 atm) from about 130° C. to about 350° C. are suitable.Widely used solvents include terpenes such as alpha- or beta-terpineolor higher boiling alcohols such as Dowanol® (diethylene glycol monoethylether), or mixtures thereof with other solvents such as butyl Carbitol®(diethylene glycol monobutyl ether); dibutyl Carbitol® (diethyleneglycol dibutyl ether), butyl Carbitol® acetate (diethylene glycolmonobutyl ether acetate), hexylene glycol, Texanol®(2,2,4-trimethyl-1,3-pentanediol monoisobutyrate), as well as otheralcohol esters, kerosene, and dibutyl phthalate. The vehicle can containorganometallic compounds, for example those based on aluminum, or boron,to modify the contact. N-Diffusol® is a stabilized liquid preparationcontaining an n-type diffusant with a diffusion coefficient similar tothat of elemental phosphorus. Various combinations of these and othersolvents can be formulated to obtain the desired viscosity andvolatility requirements for each application. Other dispersants,surfactants and rheology modifiers, which are commonly used in thickfilm paste formulations, may be included. Commercial examples of suchproducts include those sold under any of the following trademarks:Texanol® (Eastman Chemical Company, Kingsport, Tenn.); Dowanol® andCarbitol® (Dow Chemical Co., Midland, Mich.); Triton® (Union CarbideDivision of Dow Chemical Co., Midland. Mich.), Thixatrol® (ElementisCompany, Hightstown N.J.), and Diffusol® (Transene Co. Inc., Danvers,Mass.).

Among commonly used organic thixotropic agents is hydrogenated castoroil and derivatives thereof. A thixotrope is not always necessarybecause the solvent coupled with the shear thinning inherent in anysuspension may alone be suitable in this regard. Furthermore, wettingagents may be employed such as fatty acid esters, e.g.,N-tallow-1,3-diaminopropane dioleate; N-tallow trimethylene diaminediacetate; N-coco trimethylene diamine, beta diamines; N-oleyltrimethylene diamine; N-tallow trimethylene diamine; N-tallowtrimethylene diamine dioleate, and combinations thereof.

It should be kept in mind that the foregoing compositional ranges arepreferred and it is not the intention to be limited to these rangeswhere one of ordinary skill in the art would recognize that these rangesmay vary depending upon specific applications, specific components andconditions for processing and forming the end products.

Other Additives.

Other inorganic additives may be added to the paste to the extent ofabout 1 to about 30 wt %, preferably about 2 to about 25 wt % and morepreferably about 5 to about 20 wt % based on the weight of the pasteprior to firing. Other additives such as clays, fine silicon, silica, orcarbon powder, or combinations thereof can be added to control thereactivity of the aluminum and boron with silicon. Common clays whichhave been calcined are suitable. Fine particles of low melting metaladditives (i.e., elemental metallic additives as distinct from metaloxides) such as Pb, Bi, In, Ga, Sn, Sb, and Zn and alloys of each withat least one other metal can be added to provide a contact at a lowerfiring temperature, or to widen the firing window.

A mixture of (a) glasses or a mixture of (b) crystalline additives andglasses or a mixture of (c) one or more crystalline additives can beused to formulate a glass component in the desired compositional range.The goal is to reduce bowing and improve the solar cell electricalperformance. For example, second-phase crystalline materials such asBi₂O₃, Sb₂O₃, In₂O₃, Ga₂O₃, SnO, ZnO, SiO₂, ZrO₂, Al₂O₃, B₂O₃, V₂O₅,Ta₂O₅, various alumino-silicates, bismuth silicates such as12Bi₂O₃.SiO₂, 2Bi₂O₃.SiO₂, 3Bi₂O₃.5SiO₂, Bi₂O₃.4SiO₂, 6Bi₂O₃.SiO₂,Bi₂O₃.SiO₂; 2Bi₂O₃.3SiO₂; bismuth titanates such as Bi₂O₃.2TiO₂,2Bi₂O₃.3TiO₂, 2Bi₂O₃.4TiO₂, and 6Bi₂O₃.TiO₂; various vanadates such asMgO.V₂O₅, SrO.V₂O₅, CaO.V₂O₅, BaO.V₂O₅, ZnO.V₂O₅, Na₂O.17V₂O₅,K₂O.4V₂O₅, 2Li₂O.5V₂O₅; and bismuth vanadates such as 6Bi₂O₃.V₂O₅,BiVO₄, 2Bi₂O₃.3V₂O₅, and BiV₃O₉; bismuth vanadium titanates such as6.5Bi₂O₃.2.5V₂O₅.TiO₂; zinc titanates such as 2ZnO.3TiO₂; zinc silicatessuch as ZnO.SiO₂; zirconium silicates such as ZrO₂.SiO₂; and reactionproducts thereof and combinations thereof may be added to the glasscomponent to adjust contact properties. However, the total amounts ofthe above oxides must fall within the ranges specified for variousembodiments disclosed elsewhere herein.

Paste Preparation.

The paste according to the present invention may be convenientlyprepared on a planetary mixer. The amount and type of carriers utilizedare determined mainly by the final desired formulation viscosity,fineness of grind of the paste, and the desired wet print thickness. Inpreparing compositions according to the present invention, theparticulate inorganic solids are mixed with the vehicle and dispersedwith suitable equipment, such as a planetary mixer, to form asuspension, resulting in a composition for which the viscosity will bein the range of about 100 to about 500 kcps, preferably about 300 toabout 400 kcps, at a shear rate of 9.6 sec⁻¹ as determined on aBrookfield viscometer HBT, spindle 14, measured at 25° C.

Printing and Firing of the Pastes.

The inventive method of making a solar cell back contact comprises: (1)applying an Al—B containing paste to the P-side of a silicon wafer onwhich back silver rear contact paste is already applied and dried, (2)drying the paste, and (3) applying the front contact silver paste, and(4) co-firing the front contact, silver rear contact, and Al—B backcontact. The solar cell printed with silver rear contact Ag-paste, Al—Bback contact paste, and Ag-front contact paste is fired at a suitabletemperature, such as about 650-950° C. furnace set temperature; or about550-850° C. wafer temperature. During firing as the wafer temperaturerises above the Al—Si eutectic temperature of 577° C., the back contactAl and B dissolves Si from the substrate and a liquid Al—B—Si layer isformed. This Al—B—Si liquid continues to dissolve substrate Si duringfurther heating to peak temperature. During the cool down period. Siprecipitates back from the Al—B—Si melt. This precipitating Si grows asan epitaxial layer on the underlying Si substrate and forms a p+ layer.When the cooling melt reaches the Al—Si eutectic temperature theremaining liquid freezes as an Al—Si eutectic layer. A p+ layer isbelieved to provide a back surface field (BSF), which in turn increasesthe solar cell performance. The glass in Al—B back contact shouldoptimally interact with both Al (and/or Al—B materials) and Si withoutunduly affecting the formation of an efficient BSF layer.

Method of Back Contact Production.

A solar cell back contact according to the present invention can beproduced by applying any Al—B paste disclosed elsewhere herein, producedby mixing aluminum powders, boron containing materials, with the glasscompositions of Tables 1, 2, or 3, to the P-side of the siliconsubstrate pre-coated with silver rear contact paste, for example byscreen printing, to a desired wet thickness, e.g., from about 30 to 50microns. Front contact Ag pastes are then printed on the front side.

Common to the production of front contacts, back contacts and silverrear contacts is the following. Automatic screen-printing techniques canbe employed using a 200-325 mesh screen. The printed pattern is thendried at 200° C. or less, preferably at about 120° C. for about 5-15minutes before firing. The dry printed Al—B back contact paste of thepresent invention can be co-fired with the silver rear contact and thefront contact silver pastes for as little as 1 second up to about 5minutes at peak temperature, in a belt conveyor furnace in air.

Nitrogen (N₂) or another inert atmosphere may be used if desired, but itis not necessary. The firing is generally according to a temperatureprofile that will allow burnout of the organic matter at about 300° C.to about 550° C., a period of peak furnace set temperature of about 650°C. to about 1000° C., lasting as little as about 1 second, althoughlonger firing times as high as 1, 3, or 5 minutes are possible whenfiring at lower temperatures. For example a three-zone firing profilemay be used, with a belt speed of about 1 to about 4 meters (40-160inches) per minute. Naturally, firing arrangements having more than 3zones are envisioned by the present invention, including 4, 5, 6, or 7,zones or more, each with zone lengths of about 5 to about 20 inches andfiring temperatures of 650 to 1000° C.

EXAMPLES

Polycrystalline silicon wafers, used in the following examples were 243cm² and about 180 microns thick. These wafers were coated with a siliconnitride antireflective coating on the N-side of Si. The sheetresistivity of these wafers was about 1 Ω-cm. Exemplary glasses of theinvention are presented in Table 4.

TABLE 4 Exemplary Glass Compositions Glass Mole % 1 2 3 Bi₂O₃ 35.8 12.2SiO₂ 35.5 62.6 37.0 B₂O₃ 7.2 8.0 TiO₂ 5.0 18.2 V₂O₅ 8.8 Li₂O 6.1 5.4Na₂O 20.9 K₂O 21.5 6.1 8.8 P₂O₅ 0.9

Exemplary Al—B formulations in Table 5 were made with the glasses ofTable 4 plus commonly used 2-5 micron silver powders or flakes and 4-10micron aluminum (or aluminum-boron powders), Cabosil®, organic boron andtitanium solutions, Anti-Terra® 204, organic vehicles and Texanol®. Theclay is Na_(0.3)(Mg,Li)₃Si₄O₄O₁₀(OH)₂ which is calcined to drive offwater, then pulverized (ball-milled) using conventional means, andwetted at a ratio of 40 wt % calcined clay, 59 wt % terpineol, and 1 wt% Anti-Terra® 204. Anti-Terra® 204 is a wetting agent commerciallyavailable from BYK-Chemie GmbH, Wesel, Germany. Cabosil® is fumedsilica, commercially available from Cabot Corporation, Billerica Mass.Boron ethoxide and tetraethyl orthotitanate are available fromSigma-Aldrich, Dallas, Tex. Vehicle A is a blend of Texanol® (85.3%),Ethyl cellulose resin (9.5%), Thixatrol-ST (3.3%) and Triton X-100(1.9%). Vehicle B is a blend of Texan® (88%) and Thixatrol-ST (12%).

TABLE 5 Exemplary Aluminum-Boron Paste Formulations Paste (wt %) Paste APaste B Glass 1 powder 0.8 Glass 2 powder 0.8 Glass 3 powder 0.5Aluminum powder 70.44 Aluminum-Boron powder (0.2 wt % B) 77.93 Cabosil ®0.35 0.4 Boron ethoxide solution 1.5 Tetraethyl orthotitanate solution1.5 Organic Vehicle A 6 4.5 Organic Vehicle B 5 6.8 Texanol ® 8.6 2.79Anti-Terra 204 1.11 1.0 Clay 4.98 Alpha-Terpineol 5.0

The exemplary Al—B back contact pastes in Table 5 were printed on asilicon solar cell pre coated with the hack side silver/aluminum pasteCN33-451, available from Ferro Corporation, Cleveland, Ohio. Both pasteswere printed using 200 mesh screen. After drying the back contact paste,the front contact paste CN33-455, available from Ferro Corporation,Cleveland, Ohio, was printed using a 280 mesh screen with 100 micronopenings for finger lines and with about 2.8 mm spacing between thelines. The printed wafers were co-fired using a 3-zone infrared (IR)belt furnace with a belt speed of about 3 meters (120″) per minute, withtemperature settings of 780° C., 830° C., and 920° C., respectively. Thezones were 7″, 16″, and 7″ long, respectively. The fired finger widthfor the front contact Ag lines was about 120 to 170 microns and thefired thickness was about 10 to 15 microns.

A prior art “low bow” aluminum paste (commercially available FerroCN53-101) was fired side by side with an aluminum-boron paste accordingto the invention using Ampal 3510 aluminum powder. About 1.7 grams ofthe respective pastes were printed onto the silicon wafers. Electricalperformance of some of these solar cells was measured with a solartester, Model 91193-1000, Oriel Instrument Co., Stratford, Conn., underAM 1.5 sun conditions, in accordance with ASTM G-173-03. The electricalproperties of the resultant solar cells are set forth in Table 6, whichshows typical solar cell electrical properties and bowing for similarwafers for comparison of a prior art low bow Al paste and an Al—B pasteof the invention.

TABLE 6 Comparison of Al—B paste fired back contact with prior art Alback contact. Jsc Bowing Paste (mA/cm²) Voc (mV) FF Eff (%) (mm) Paste A(Al/B) 33.8 605 0.72 14.8 0.5-0.8 (invention) Low bow Al paste 33.2 6000.71 14.1 1.2-1.5 (prior art)

In Table 6, Jsc means current density; Voc means open circuit voltagemeasured at zero output current; Efficiency (Eff) is known in the art.

Additional advantages and modifications will readily occur to thoseskilled in the art. Therefore, the invention in its broader aspects isnot limited to the specific details and illustrative example shown anddescribed herein. Accordingly, various modifications may be made withoutdeparting from the spirit or scope of the general invention concept asdefined by the appended claims and their equivalents.

The invention claimed is:
 1. A solar cell comprising two separatelayers, a silicon layer and a back contact layer deposited over a backsurface of the silicon layer, wherein the back contact layer comprises ap⁺ layer with an active peak doping concentration of (Al+B) of about10¹⁹ to about 10²⁰ atoms per cubic centimeter, the back contact layermade from a mixture, wherein, prior to firing, the mixture comprises: a.at least one aluminum source providing aluminum in an amount of about 50wt % to about 85 wt % of the mixture, b. at least one boron sourceproviding boron in an amount of about 0.25 wt % to about 20 wt % of themixture, and c. about 0.1 to about 10 wt % of a glass component,including (i) a first glass composition, wherein the first glasscomposition comprises
 1. about 15 to about 75 mole % PbO and
 2. about 5to about 50 mole % of (B₂O₃+SiO₂), and (ii) a second glass compositioncomprising
 3. about 30 to about 40 mole % (Li₂O+Na₂O+K₂O),
 4. about 2 toabout 26 mole % TiO₂, and
 5. about 5 to about 75 mole % (B₂O₃+SiO₂),wherein the first and second glass compositions are present in a weightratio of about 5:1 to 1:5.
 2. The solar cell of claim 1, wherein thefirst glass composition further comprises about 1 to about 20 mole % ofa trivalent oxide of an element selected from the group consisting of anelement having an atomic number of from 57 to 71, Al, Ga, In, Sc, and Y,and combinations thereof.
 3. The solar cell of claim 1, wherein thefirst glass composition comprises: a. about 50 to about 65 mole % PbO,b. about 24 to about 45 mole % (B₂O₃+SiO₂), and c. further comprises0.1-8 mol % Al₂O₃, and d. further comprises at least one oxide, whereinthe content of said oxide does not exceed the indicated amount in mol %,selected from the group consisting of ZnO (25%) and ZrO₂ (5%).
 4. Thesolar cell of claim 1, wherein the first glass composition furthercomprises ZnO, provided the ZnO content of the first glass compositiondoes not exceed 55 mole %.
 5. The solar cell of claim 1, wherein thefirst glass composition further comprises (Li₂O+Na₂O+K₂O), wherein the(Li₂O+Na₂O+K₂O) content in the first glass composition does not exceed40 mole %.
 6. The solar cell of claim 1, wherein the first glasscomposition further comprises (TiO₂+ZrO₂), wherein the (TiO₂+ZrO₂)content in the first glass composition does not exceed 20 mole %.